fix(MHPL-615): cap QUIC UDP payload size for Cloudflare Spectrum

[alepaez]

Problem

Players intermittently disconnect while playing on the Hytale server when it sits behind Cloudflare Spectrum. Confirmed: connecting directly to the origin is stable; through Spectrum it drops. (MHPL-615)

Root cause

Hytale runs on QUIC over UDP (Netty + quiche, ALPN hytale/2, UDP 5520). Two facts collide:

1. Cloudflare Spectrum does not fragment UDP — it drops any datagram too large to forward at its edge (Spectrum limitations). The edge→origin leg, minus QUIC/UDP/IP overhead on an anycast path with ICMP "Packet Too Big" filtered, has a lower effective MTU than a normal path.
2. The client-facing QUIC codec invited large datagrams. It never set maxRecvUdpPayloadSize, so it advertised quiche's default max_udp_payload_size = 65527 to clients (letting the client grow its send size toward its local ~1450 MTU), and ran discoverPmtu(true) (probing its own sends upward). IP_DONTFRAGMENT=true on the sockets removes any fallback.

When a grown datagram exceeds the Spectrum path MTU — most often during large-data bursts (voxel chunks / world-map / voice) or after an anycast/route change — it is silently blackholed. QUIC must detect the blackhole and fall back; during that loss window an active session stalls, and with a 60 s idle timeout and no keepalive a bad-enough stall becomes a full disconnect. Small steady-state packets fit under the limit, which is why it's intermittent rather than constant.

Fix

Cap the QUIC datagram size so packets stay within the Spectrum-forwardable size, and stop PMTU probing — both driven by new config keys so the value can be tuned per deployment without a rebuild:

Key	Default	Effect
max-udp-payload-size	1200	Sets maxRecvUdpPayloadSize (advertised to the client → caps client→proxy, the direction Spectrum blackholes) and maxSendUdpPayloadSize (caps proxy→client). Must be ≥ 1200.
discover-pmtu	false	Drives discoverPmtu(...). Off behind Spectrum; set true only for direct (non-Spectrum) UDP exposure.

Default ships at 1200 — QUIC's universal floor, guaranteed deliverable on any path — so the first test deploy is maximally safe.

How to test

1. Build server.jar from this branch and deploy it into hytale-proxy (the deployed jar is committed there; this branch is what it's built from).
2. Play through Spectrum with the default max-udp-payload-size = 1200. Disconnects should stop.
3. To recover throughput, bump max-udp-payload-size in hytale-proxy/config.toml (no rebuild — it's a config key) to 1252 → 1350, re-testing stability at each step. Back off one step at the first sign of drops.
4. -Dhyproxy.debugBytes=true can be used to watch frame sizes during testing.

Verification

• Compiles cleanly against Netty QUIC 4.2.9.Final with JDK 25 (:common + :proxy compileJava SUCCESSFUL). No unit tests exist in this repo; live Spectrum testing is the real verification.

Notes / scope

• Client-facing codec only. The proxy→backend leg (InboundForwardingPacketHandler) is intra-cluster and never crosses Spectrum, so it's untouched.
• Backwards compatible. Existing config.toml files without the new keys fall back to the defaults (1200 / false).
• Base branch: targets alexandresequeira/mhg-1132-update-protocol because main is frozen (identical to upstream SantioMC/hyproxy, no Hytale 0.5.5 support) and the deployed proxy is built from the MHG-1132 branch.
• Follow-up (not in this PR): the 60 s idle timeout with no keepalive and activeMigration(false) are secondary Spectrum-robustness levers worth revisiting if any drops remain after the MTU cap.